Adolescents engaging in non-suicidal self-injury (NSSI) have been reported to show altered autonomic nervous system (ANS) function, indexed by decreased heart rate variability (HRV) and increased heart rate (HR). Preliminary findings in adolescents with borderline personality disorder (BPD) traits suggest, that improvement in ANS function is longitudinally associated with clinical improvement and that ANS activity predicts clinical outcome. Existing studies, however, are limited by small sample sizes and short follow-ups. N = 227 help-seeking adolescents with NSSI disorder who presented to an early intervention service for BPD participated in baseline and two yearly follow-up assessments (NFU1 = 81, NFU2 = 37), including comprehensive clinical diagnostics as well as recordings of resting electrocardiography. Associations between HR, HRV and clinical outcomes of interest (i.e., NSSI frequency, depression severity, number of BPD criteria and global functioning) were examined using structural equation modelling. While multivariate models showed no evidence for an association between HRV or HR and any of the clinical outcomes, there was evidence from models including cross-lagged effects, that HRV and HR predicted depression severity, number of BPD criteria (HRV only) and global functioning at subsequent assessments. This effect was not observed for NSSI frequency. Improvements in ANS function, indexed by an increase in HRV and decrease in HR, seem to precede the improvement of clinical symptoms in adolescents engaging in NSSI. Findings have clinical implications, suggesting that targeting ANS function as adjuvant treatment in adolescents engaging in NSSI is warranted, and routine monitoring of ANS function my guide clinical decision making.
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